As Leonid mentions in [one of his answers](http://mathematica.stackexchange.com/a/109917/34008) one of the methods of managing complexity is using [Domain Specific Languages (DSL's)](https://en.wikipedia.org/wiki/Domain-specific_language). In this answer I will provide links to documents, packages, blog posts, and discussions of creating and utilizing DSL's in *Mathematica*.

For a 2.5 minutes introduction see [this video](http://www.wolfram.com/broadcast/video.php?c=400&v=1470) between 25:00 and 27:30.

**When to apply DSL's**

Here are some cases for applying DSL.

1. When designing conversational engines.

2. When there are too many tuning options and usage scenarios of the developed algorithms.

* For example, we have a bunch of search, recommendation, and interaction algorithms for a dating site. A different department designs interactive user interfaces for these algorithms. We make a natural language DSL that invokes the different algorithms according to specified outcomes.


3. When designing an API for a collection of algorithms.

* Just designing a DSL can bring clarity of what signatures should be in the API.

* `NIntegrate`'s `Method` option was designed and implemented using a DSL. See [this video](http://www.wolfram.com/broadcast/video.php?c=400&v=1470) between 25:00 and 27:30.

**Designing DSL**

1. Decide what kind of sentences the DSL is going to have.

* Are natural language sentences going to be used? 

* Are the sentences mathematical in nature?
	
2. Prepare, create, or accumulate sentences.

* In some cases using [Morphological Analysis](https://en.wikipedia.org/wiki/Morphological_analysis_(problem-solving)) can greatly help for coming up with use cases and the corresponding sentences.   	

3. Create a context free grammar that describes the sentences. (Or a large subset of them.)

* At this stage I use exclusively [Extended Backus-Naur Form (EBNF)](https://en.wikipedia.org/wiki/Extended_Backus–Naur_Form).

4. Program parser(s) for the grammar.

* I use most of the time [functional parsers](https://en.wikipedia.org/wiki/Parser_combinator).

* The package [FunctionalParsers.m](https://github.com/antononcube/MathematicaForPrediction/blob/master/FunctionalParsers.m) provides a *Mathematica* implementation of this kind of parsing.

* The package can automatically generate parsers from a grammar given in EBNF. (See the coding example below.)

5. Program an interpreter for the parsed sentences. 

* At this stage the parsed sentences are hooked to the algorithms of the problem domain.

* In *Mathematica* this can be done fairly easy using `Case` to pick grammar rule wrappers.

6. Test the parsing and interpretation.

See the example below illustrating steps 3-6.

**Introduction to using DSL's in Mathematica**

1. This blog post of mine ["Natural language processing with functional parsers"](https://mathematicaforprediction.wordpress.com/category/functional-parsers/).

2. This detailed slide-show presentation ["Functional parsers for an integration requests language grammar"](https://github.com/antononcube/MathematicaForPrediction/blob/master/Documentation/Functional%20parsers%20for%20an%20integration%20requests%20language%20grammar.pdf) shows how to use the package [FunctionalParsers.m](https://github.com/antononcube/MathematicaForPrediction/blob/master/FunctionalParsers.m) over a small grammar.


**Advanced example**

The blog post ["Simple time series conversational engine"](https://mathematicaforprediction.wordpress.com/2014/11/29/simple-time-series-conversational-engine/) discusses the creation (design and programming) of a simple conversational engine for time series analysis (data loading, finding outliers and trends.)

Here is movie demonstrating that conversation engine: [http://youtu.be/wlZ5ANglVI4](http://youtu.be/wlZ5ANglVI4).

**Other discussions**

1. Small part from 17:30 to 21:00 of the WTC 2012 ["Spatial Access Methods and Route Finding"](http://www.wolfram.com/broadcast/video.php?sx=Spatial%20Access%20Methods%20and%20Route%20Finding&v=35) presentation shows a DSL for points of interest queries.

2. The [answer](http://mathematica.stackexchange.com/questions/49052/css-selectors-for-symbolic-xml/49053#49053) of the MSE question ["CSS Selectors for Symbolic XML"](http://mathematica.stackexchange.com/questions/49052/css-selectors-for-symbolic-xml) uses [FunctionalParsers.m](https://github.com/antononcube/MathematicaForPrediction/blob/master/FunctionalParsers.m).

**Coding example**

This example is for the steps 3-6 of the second section.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/WDvvR.png